Various metal oxides are used for a variety of applications. Indium oxide is a well-known material and is used as a transparent electrode material needed for liquid crystal displays and the like.
Some metal oxides have semiconductor characteristics. Metal oxides having semiconductor characteristics are a kind of compound semiconductor. The compound semiconductor is a semiconductor formed by two or more kinds of atoms bonded together. In general, metal oxides serve as insulators; however, it is known that metal oxides serve as semiconductors depending on the combination of elements included in the metal oxides.
For example, it is known that some metal oxides such as tungsten oxide, tin oxide, indium oxide, and zinc oxide have semiconductor properties.
A technique is disclosed in which a transistor is manufactured using zinc oxide or an In—Ga—Zn-based oxide and the transistor is used as a switching element or the like of a pixel of a display device (see Patent Documents 1 and 2).
Transistors including oxide semiconductors have higher field-effect mobility than transistors including amorphous silicon. Therefore, driver circuits of display devices and the like can be formed using the transistors including oxide semiconductors.
Display devices include EL display devices, electronic paper, and liquid crystal display devices. Among these, active matrix EL display devices capable of displaying high-resolution images are particularly attracting attention. In an active matrix EL display device, where a plurality of switching elements (also referred to as pixel transistors) are arranged in pixels, voltage is applied to a light-emitting element electrically connected to at least one of the switching elements, whereby electrons and holes are separately injected from a pair of electrodes into a layer containing a light-emitting organic compound, and current flows. The carriers (electrons and holes) are recombined, and thus, the light-emitting organic compound is excited. The light-emitting organic compound returns to a ground state from the excited state, thereby emitting light. Owing to such a mechanism, this light-emitting element is referred to as a current-excitation light-emitting element.
The range of uses of such an active matrix display device is expanding, and demands for larger screen size, higher definition, and higher aperture ratio are increasing. In addition, it is demanded that a production method of the active matrix display device offer high productivity and reduced production cost.
It is important for a pixel transistor, which is a switching element, to have a sufficiently low off-state current (drain current which flows when the transistor is off). Furthermore, a sufficiently low off-state current contributes to lower power consumption.
As a material for a semiconductor layer of a pixel transistor, a silicon material such as polysilicon or amorphous silicon, an oxide semiconductor, or the like is used.
To reduce manufacturing cost of display modules, it is preferable to manufacture the display modules using a large-sized substrate with an insulating surface (e.g., glass substrate or plastic substrate).
A laser irradiation apparatus, which is used for a crystallization process or the like of polysilicon, has limitations on irradiation area per laser light source and irradiation area per unit time. Such limitations make it difficult to process a large-sized substrate with an insulating surface in a short time.
A manufacturing process of a transistor including an oxide semiconductor does not use a laser irradiation apparatus. Furthermore, a transistor including an oxide semiconductor has higher field-effect mobility than a transistor including amorphous silicon, whose manufacturing process also does not use a laser irradiation apparatus. Therefore, driver circuits of display devices and the like can be formed using a transistor including an oxide semiconductor.